Metalloproteins containing manganese in a redox-active role are involved in a variety of physiologically important reactions of dioxygen metabolism. These include, amongst others, a superoxide dismutase that detoxifies superoxide radicals to O2 and peroxide, a catalase that disproportionates peroxide to O2 and H2O, and perhaps the most complex and important, the Mn-containing oxygen-evolving complex (Mn-OEC) that is involved in the oxidation of water to dioxygen in photosystem II. The Mn-OEC generates almost all of the dioxygen that supports aerobic life, and it is abundant in the atmosphere because of its constant regeneration by the oxidation of water. The light-induced oxidation of water to dioxygen is one of the most important chemical processes occurring on such a large scale in the biosphere. The water-oxidation reaction involves removal of four electrons, in a stepwise manner by light-induced oxidation, from two water molecules by the Mn-OEC to produce a molecule of oxygen. Central questions that need to be resolved and the overall objective of this proposal are as follows: 1) Characterize the structure and the changes of the oxo-bridged heteronuclear Mn4Ca complex as it advances through the enzymatic cycle, 2) Elucidate the structural and functional role of the cofactors Cl- and Ca2+, 3) Determine the oxidation states and electronic structure of the Mn complex in the four intermediate S-states, 4) Determine the mechanism of water oxidation and oxygen evolution. The interplay between X-ray spectroscopy and EPR has played an essential role in our understanding of the structural and mechanistic aspects of O2 evolution. The samples for X-ray spectroscopy will be characterized by EPR. The structural changes of the Mn complex as it advances through the enzymatic cycle are determined by high-resolution XAS methods using samples prepared by flash illumination, and single- crystals of PS II. The oxidation states and electronic structure of the Mn complex are determined by Mn K- and L-edge, K? emission, and X-ray resonant Raman spectroscopies. PUBLIC HEALTH RELEVANCE: Several manganese enzymes are involved in the process of oxygen metabolism, and one of the most complicated and least understood is the multinuclear manganese cluster in the oxygen-evolving complex, which is the only enzyme that is capable of generating O2 from H2O. This proposal is directed towards the determination of the structure and mechanism of this critically important manganese enzyme.